Rugged fiber optic array

ABSTRACT

An array of fiber optic hydrophones or geophones is formed by winding of optical fiber around a continuous, yet flexible cylindrical core. The cylindrical core contains an elastomer filled with a specified percentage of voided plastic microspheres. The elastomer provides the necessary radial support of the optical fiber, and with the included voided microspheres, provides sufficient radial compliance under acoustic pressure for proper operation of the hydrophone. The cylindrical core can be made in very long sections allowing a plurality of fiber optic hydrophones to be wound onto it using a single optical fiber, with individual hydrophone elements separated by integral reflectors such as Fiber Bragg Gratings (FBSs). The center of the core may include a strength member and a central hollow tube for the passing of additional optical fibers. The aforementioned hydrophone array is then packaged within a protective outer coating or coatings as required for the specified application.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser.No. 60/416,007, filed Oct. 4, 2002, U.S. Provisional Application Ser.No. 60/463,295, filed Apr. 16, 2003, and U.S. Provisional ApplicationSer. No. 60/465,150, filed Apr. 24, 2003, the entirety of which areexpressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an improved design andconstruction technique for fiber optic hydrophones and hydrophonearrays. More specifically, the present invention comprises a fiber optichydrophone that has a continuous solid, yet compliant, elastomer core.In some embodiments, plastic microspheres have been added to theelastomer core to provide increased acoustic compliance.

2. Description of Related Art

There are many occasions when it is necessary to detect acoustic signalsin an underwater environment. For example, geologic exploration is oftencarried out by setting small explosives below the ocean's, or other bodyof water's, surface, detonating the explosives, and then detecting theresulting acoustic signals to determine the structure of variousfeatures on or under the sea floor. Additionally, there is a need to beable to detect acoustic signals, such as the sounds emitted by ships,submarines, fish or other animals, that are transmitted under water.

Generally, where acoustic signals need to be sensed or detected in anunderwater environment, sensors called hydrophones are used. In manyinstances, multiple hydrophones are joined together with a specifiedspacing between each hydrophone to form an array of hydrophones. Sucharrays of multiple hydrophones are particularly useful compared to useof single hydrophones where it is necessary to determine the directionthe acoustic signals are coming from, or to provide increasedsensitivity so as to improve the likelihood of detecting faint acousticsignals.

Conventional hydrophone arrays consist of a series of many piezoelectricelements, or sensors, each of which produces a voltage proportional tothe intensity of acoustic signals incident upon the hydrophone. Typicalhydrophones available for use in such arrays at present have variouscircuitry or other electronics associated with the sensor elementslocated at each sensor in hydrophone. These associated circuits are usedfor amplification, filtering, digitization, multiplexing and the like ofthe signals produced by the piezoelectric sensors. Because theseadditional circuits or electronics are necessarily located underwater,the circuits and electronics are exposed to harsh conditions, such asextreme pressure due to the depth the hydrophone is deployed, or waterleakage into the hydrophone housing. To protect the circuits,hydrophones typically include hermetically sealed armored housings. Ifthe circuitry does fail, however, repair of the hydrophone requires thatthe hydrophone, or, as is typically the case, an entire array or portionof an array, may need to be retrieved from its underwater deployment fordiagnosis and repair. Because such repairs are costly and timeconsuming, and may require the use of specialized vessels and equipmentto retrieve the damaged hydrophones, there is a need for a more robustacoustic sensor.

One system that provides improved robustness and reliability uses fiberoptic sensors as the sensor element in the hydrophone. Such fiber opticsensors typically use optical fiber wrapped in a high precision windingpattern around compliant, air-backed mandrels as the sensing medium.This arrangement is advantageous in that no additional circuitry orelectronics are required at the location of the sensor, making the fiberoptic sensors inherently more reliable than other types of conventionalhydrophones used in hydrophone arrays.

In a fiber optic sensor, light is sent from a source located in arelatively benign environment through the optical fiber to the sensor.Acoustic pressure waves present in the water dynamically strain thefiber, resulting in a shift in the phase of the light transmitted in theoptical fiber. The phase shifted light is compared to a referencesignal, creating an interference pattern. The resulting light signal isthen sent to an interrogator, which converts the light to an electricalsignal for demodulation.

There are several shortcomings associated with presently availablehydrophone arrays that use fiber optic sensors as the sensor element.One disadvantage of presently available fiber optic sensors is that thefiber generally is wrapped around discrete, hollow mandrels that arestiff enough to withstand the hydrostatic pressure requirements ofdeploying hydrophones under water, yet are compliant enough so that theacoustic pressure waves in the water can dynamically deform the mandrel,thereby straining the optical fiber resulting in a phase shift of thelight transmitted through the fiber. Accordingly, the mandrels must beformed into sealed, relatively hard plastic or thin metal hollowcylinders that are leak proof against water under the requiredhydrostatic pressure. In general the mandrels used in presentlyavailable fiber optic sensors are stiff and unbendable. This isdisadvantageous in that it is useful to be able to manufacture fiberoptic sensor arrays in long continuous sections, and to store sucharrays on circular drums, from which the fiber optic sensor array may bedeployed and retrieved, and such long sections need to be flexible.

To facilitate the flexibility needed to store the fiber optic sensorarrays on a circular drum, the mandrels must be made into shortcylindrical pressure vessels, such as, for example, capped tubes, withflexible links between the capped tubes to form long continuous bendablesections. A considerable amount of labor must be used to assemble theoptic fiber wrapped on the air-backed mandrels into interferometers.This laborious process includes preparing the optical fiber, splicingand recoating the optical fiber, dressing the optical fiber,mechanically assembling the pressure vessels, and sealing and testingthe vessels and optical fiber to ensure that the resulting assembly iswater tight and functions as desired.

As fiber is wound around the long continuous, bendable sections, greatcare must be exercised to ensure that the mandrel/flexible linkinterfaces do not have any sharp or uneven surfaces, and do notseparate, shift, or deform under pressure or tension, which will breakthe optical fiber. In addition, array strength members and extra opticalfibers often must be placed along the outer surface of the continuoussection, leading to optical fiber damage during reeling/unreelingoperations as a result of friction, bending, crushing, and the like.

An additional shortcoming with air-backed mandrel optical fiberhydrophones is that such devices have a flat optical phase response toacoustic input as a function of acoustic frequency. This isdisadvantageous in acoustic frequency ranges that contain unwantedacoustic signals, such as noise caused by fish, whales or other soundsource, whose presence limits the dynamic range of the overall systemunless very high sample rates are used by the system electronics tointerrogate the sensors to allow signal analysis and canceling of thenoise.

The spaces between and around the mandrels used for the fiber opticsensors may also be filled with a liquid in an attempt to provideimproved acoustic coupling and thus sensor sensitivity as well as toimprove or control the buoyancy of the array. Such construction may bedisadvantageous because such liquid-filled fiber optic sensor arrays aresusceptible to puncture during deployment and reeling operations as wellas during normal operation, and leakage of the fluid typically resultsin failure of the array. Moreover, where a fluid such as kerosene orkerosene-like liquids or other possibly environmentally hazardousmaterial is used, leakage of the fluid can contribute to unwantedpollution.

What has been needed, and heretofore unavailable, is a reliable, robustfiber optic acoustic sensor that has eliminates the disadvantages ofair-backed or fluid filled arrays, yet provides for increasedsensitivity and ease of manufacture. The present invention satisfiesthese and other needs.

SUMMARY OF THE INVENTION

The present invention overcomes many of the shortcomings associated withfiber optic hydrophone arrays manufactured in accordance with prior artmethods.

The present invention eliminates the requirement for rigid, air-backedmandrels for the fiber optic hydrophones. Instead, long cylindricalsegments of a continuously flexible elastomer form the base of thehydrophone structure. The optical fiber is wound onto the one-piecesegments that are completely free of the rigid mandrel/flexible linkinterfaces and their associated problems. Compliance is provided tohydrophone through the use of voided plastic microspheres dispersedwithin the elastomer substrate. The hardness of the microspherecontaining elastomer is controlled to meet the required acousticsensitivity and the acoustic sensitivity versus depth requirements of aparticular application.

One advantage of using an elastomer substrate is that the dynamicproperties of the elastomer, such as, for example, bulk modulus andPoisson's ratio, may be altered as necessary to tailor the mechanicalresponse of the sensor for an application. Such tailoring is useful, forexample, to achieve mechanical roll-off of undesired (that, is, out ofband) frequency response. The ability to suppress selected frequencyranges provides for limiting the bandwidth of the fiber optic sensors,reducing demands on the demodulation electronics, which historicallyrequire, using prior art sensors, sample rates that are orders ofmagnitude higher than that required to satisfy Nyquist criteria in orderto maintain large dynamic ranges.

Further, replacing the individual sealed rigid mandrels with a flexiblepolymer allows the center core of the hydrophone or geophone segment tobe used for passing extra optical fibers and/or a central strengthmember. Thus the central core of the hydrophone or geophone segment canbe constructed similar to a standard high strength fiber optictelecommunications cable with the optical fiber protected in a thinmetal tube surrounded by a rugged strength member and a tough outerjacket. Such a cable is inherently designed to withstandreeling/unreeling without stressing the optical fiber. In one embodimentof the present invention, the void filled elastomer is extruded on theoutside of the core of the cable structure in a concentric fashion,thereby forming the base upon which to wind the optical fiber to form ahydrophone or array of hydrophones.

In another aspect of the present invention, the hydrophones of thepresent invention may be disposed within a hollow tube, with theremaining space within the tube filled with a liquid, as in a standardtowed hydrophone array, or a low shear polymer compound that does notflow. The purpose of this filler is to isolate the hydrophones fromlongitudinal shear waves which cause noise in the output of the array.

In other aspects of the present invention, the outer surface of thehydrophone may be formed from a tough, abrasion resistant elastomericcover, which provides turbulent boundary layer noise rejection. In oneembodiment, microspheres may be added to the low shear polymer and/orthe elastomeric cover as needed to adjust buoyancy of the hydrophonearray. This embodiment is particularly advantageous in that thehydrophone may be used as is, that is, the fiber optic hydrophone may betowed without needing to be encased within a fluid filled tube, as istypical for fiber optic hydrophone arrays presently used.

In another aspect of the present invention, the hydrophone of thepresent invention may be constructed without the abrasion resistantcover. In one such embodiment, the hydrophone may be mounted within afluid filled tube, as is typical in present towed hydrophone arrays.

In yet another aspect, the present invention is embodied in acontinuous, flexible cylindrical device for detecting acoustic signals,comprising a flexible core including an acoustic substrate, an opticalfiber wound around the acoustic substrate, and at least one periodicrefractive index perturbation formed in the optical fiber. In anotheraspect, the acoustic substrate contains a plurality of voids. In still afurther aspect, the voids are formed by hollow microspheres, which maybe formed from a compliant material.

In still another aspect of the present invention, the flexible coreincludes a hollow tube for providing a passageway through the core. Inone aspect, the flexible core includes a strength member for providingtensile strength to the core to resist stretching or breaking of thecore during deployment, retrieval or use. In another aspect, theflexible core includes a strength member surrounding the hollow tube forproviding tensile strength to the core to resist stretching or breakingof the core during deployment, retrieval or use.

Still another aspect of the present invention includes an intermediatejacket disposed between the metal tube and the central strength member.In one aspect, the jacket is disposed over the strength member.

In another aspect, the present invention includes an acoustic substrateincluding an elastomeric material having a selected dynamic property forlimiting the sensor frequency response to within a desired range offrequencies.

In still another aspect, the optical fiber is wound under tension toform at least one optical hydrophone. In yet a further aspect, thepresent invention includes an embodiment wherein the optical fiber iswound under tension to form a plurality of optical hydrophones, witheach of the hydrophones separated by a periodic refractive indexperturbation. In one aspect, the periodic refractive index perturbationis a Bragg grating; in another aspect, the periodic refractive indexperturbation is a long period grating.

In another aspect, the present invention may include a layer of tapedisposed around the acoustic substrate under the optical fiber which mayhave a low coefficient of friction relative to a coefficient of frictionof the fiber. The tape may be formed from Teflon or the like.

In still another aspect, the present invention may include a filler rodinter-wound on the acoustic substrate with the optical fiber such thatthe filler rod is disposed approximately parallel to the optical fiber.In one aspect the filler rod may be formed of nylon. In another aspect,the filler rod has a diameter equal to or larger than a diameter of theoptical fiber. In yet another aspect, the filler rod and optical fiberare inter-wound around the acoustic substrate so that there is a spacebetween adjacent turns of the wound optical fiber, the space beingfilled with a compliant material, such as a thermoplastic elastomer ordepolymerized rubber.

In a further aspect, the present invention may include a tape layerdisposed around the inter-wound filler rod and optical fiber. In oneaspect, the tape layer is formed from a material having a lowcoefficient of friction relative to the optical fiber, such as Teflon orpolyimide polymer.

Another aspect of the present invention an external layer formed from anelastomer, which may be void filled. In another aspect, the presentinvention may include an outer tube in which the flexible core andoptical fiber are disposed, there also being a space between the outertube and the flexible core and optical fiber; and a material disposedwithin the space, the material for coupling acoustic signals from theouter tube to the flexible core and optical fiber. In one aspect, thematerial is a fluid, such as Isopar or Norpar (ExxonMobil Chemical Co.);in another aspect, the material is a low shear modulus polymer.

In a still further aspect, the present invention may include an outerelastomeric layer disposed on the flexible core and optical fiber, theelastomeric layer including hollow microspheres dispersed through theelastomeric layer to adjust buoyancy.

Other features and advantages of the invention will become apparent fromthe following detailed description, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, the featuresof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of one embodiment of the present inventiondepicting an array of fiber optic acoustic sensors mounted on a flexiblesub-core assembly and encased in a protective sheath.

FIG. 1B is a cross-sectional view of the embodiment of the presentinvention depicted in FIG. 1A.

FIG. 2 is an enlarged perspective view of a portion of the embodiment ofFIG. 1 illustrating the spacing of fiber optic hydrophone elements woundupon the flexible sub-core assembly.

FIG. 2A is an enlarged perspective view of a portion of the embodimentof FIG. 1 illustrating the spacing of fiber optic hydrophone elementswound upon the flexible sub-core assembly and showing the use of a ringof low shear or high loss material disposed between adjacent windings ofthe fiber optic to decouple mechanical motion between the windings.

FIG. 3 is a perspective view, partially in cross-section, of anembodiment of the present invention where the hydrophone array isdisposed within a fluid filled outer protective covering.

FIG. 4 is a perspective view, partially in cross-section, of anotherembodiment of the present invention where the hydrophone array isdisposed within an outer protective covering and where the space betweenthe hydrophone array and the outer covering is filled with a low shearsolid material.

FIG. is a schematic view illustrating deployment of a hydrophone arrayin accordance with one embodiment of the present invention down a wellbore.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the exemplary drawings, which illustrate, by way ofexample only, embodiments of the present invention, the presentinvention is generally embodied in a structure and method for formingthat structure that includes a relatively flexible core through whichextends an optical fiber. A series of reinforcing and protection layersare further would around the core, one layer of which includes windingsof an optical fiber in which are formed one or more gratings, such aslong period or Bragg gratings. The entire structure may further besurrounded by an outer jacket. In some embodiments, the resultingcable-line structure may be extended through a protective cylinderformed from a material such as glass, metal, polymer or other materialas needed to provide additional protection to the sensor of the presentinvention depending upon the environment in which the sensor is to bedeployed.

FIG. 1 is an overall view of one embodiment of a solid fiber optichydrophone array 10 in accordance with the present invention. A coresub-assembly 18 is formed using various layers of materials as describedbelow, and then wrapping an optical fiber 17 around the coresub-assembly.

In the embodiment depicted in FIG. 1, the core sub-assembly, startingfrom the innermost portion of the core, includes a durable hollow tube11. This hollow tube may be formed from thin-walled metal tubing, suchas, for example, but not limited to, 316 stainless steel, or otherrelatively flexible material suitable for use in the environment inwhich the hydrophone is to be deployed. The hollow tube 11 provides aspace that extends throughout the entire length of the hydrophone orhydrophone array that may be used to house additional optical fibers 12,wires, or other communications means, that may be needed to send orreceive signals from sensors or other equipment located downstream ofthe hydrophone or hydrophone array. Such a provision allows for a numberof hydrophone arrays to be deployed using the same basic cable-likestructure.

Hollow tube 11 my be surrounded by an intermediate elastomeric layer 13.This intermediate elastomeric layer may provide protection to the hollowtube, and may also act as an adhesive layer to aid in forming andattaching additional layers to the hollow tube 11. Typical materialsthat may be used are, for example, polyurethane and polyethylene.

Surrounding the elastomeric adhesive layer 13 is a strength member 14made of metal wires or synthetic or natural fibers. For example, in oneembodiment, the strength member layer 14 may be formed by closelywinding a dense layer of synthetic fibers such as, for example, aramidfibers or Vectran, a product of Celanese Acetate LLC. The strengthmember limits the tensile strain transmitted to the hydrophone arraystructures during manufacture and deployment of the array to preventfailure of the assembly under the tensile strain forces experiencedduring deployment, retrieval and operational loading of the arrayassembly. It will be understood by those skilled in the art that otherhigh strength natural or synthetic fibers may be used, depending on thedesign and operational specifications desired for a particularapplication. Moreover, in an embodiment of the present invention,electrical wires, such as twisted pairs of wires, may be wound aroundthe hollow tube 11 before strength member 14 is added to the coresub-assembly.

The strength member 14 may be surrounded by an elastomer layer 15.Elastomer layer 15 encapsulates the inner layers of the coresub-assembly and also provides for maintenance of the radial uniformityof the strength member 14 and also may also provide a means for dampingand isolating the hydrophone from vibration transmitted along thestrength member 14. In some embodiments, elastomer layer 15 issurrounded by a layer 16 formed from a solid compressible material suchas an elastomer. In one embodiment, the elastomer forming layer 16 maybe a polyurethane or silicone rubber. In another embodiment, voids maybe formed introduced within the solid compressible material with amaterial such as, for example, Expancel (Azko Nobel) closed cellpolyethylene foam and the like.

Once layers 13, 14, 15 and 16 have been formed around hollow tube 11,the resulting core sub-assembly 18 is in the form of a long, continuous,flexible cylinder, which serves as the mounting base for the opticalsensor fiber 17. Using methods known in the art, such as winding with astandard cable manufacturing taping head, the optical sensor fiber 17 iswrapped under a selected amount of tension, on the order of 100 grams,onto core sub-assembly 18 such that it remains under tension under allexpected operating conditions of the hydrophone. Wrapping the fiberoptic sensor 17 around core sub-assembly 18 in this manner ensures thatan acoustic wave impacting the sensor will uniformly strain the opticalsensing fiber. As shown in FIG. 2, fiber Bragg gratings, or long periodgratings, may be incorporated into the optical sensor fiber 17 atappropriate intervals to form one or more acoustic sensors along thelength of the hydrophone assembly.

As is known in the art, fiber Bragg gratings may be incorporated into anoptical fiber using a variety of methods. One such method, for example,is described in U.S. Pat. No. 6,222,973, Fabrication of Refractive IndexPatterns in Optical Fibers having Protective Optical Coatings, issuedApr. 24, 2001, the subject matter of which is incorporated herein byreference in its entirety.

Once the optical fiber 17 has been wound on the core sub-assembly, anadhesive may be applied to hold the optical fiber in position on thecore sub-assembly. The adhesive may also be applied during the windingprocess. Prior to winding the optical fiber 17 onto the coresub-assembly 18, the core sub-assembly 18 may be wrapped or coated witha layer 19 of material, which may be in the form of a tape, such as, forexample, but not limited to, Teflon (DuPont de Nemours Co.), polyimideor other suitable material, having a low coefficient of friction withrespect to the jacket of the optical fiber 17. The addition of lowfriction layer 19 ensures that optical fiber 17 can move with respect tooverlying layers during bending of the hydrophone 10, reducing oreliminating the introduction of longitudinal strain onto the opticalfiber 17 that may result in tensile failure of optical fiber 17.

The optical fiber 17, including any gratings formed therein, may beuncoated, or it may be coated prior to winding with a metallic ornon-metallic materials, depending on the needs of the particularapplication in which the hydrophone is to be used. Where a coating isapplied, the optical fiber may be coating using known processes, such aspressure or tubing extrusion. Coating the optical fiber 17 prior towinding typically improves the acoustic sensitivity of the resultantsensor.

In an embodiment of the present invention where the optical fiber 17 iscoated with a solid elastomer, or an elastomer that has been modified toinclude voids dispersed within the elastomer coating, the acousticsubstrate of the hydrophone, layer 16, may be formed from a stiffermaterial than would otherwise be appropriate. For example, layer 16 maybe formed from a polymer having a relatively higher elastic modulus of,for example, on the order of 80 or greater Shore A hardness, such as asuitable silicone polymer, or an incompressible polymer such as, forexample, unfilled polyurethane or polyethylene. Forming layer 16 fromsuch a material may be advantageous where reduced sensitivity of thefiber optic sensors is required, such as to hydrostatic pressures causedby deep deployment of the sensors in water, such as in the ocean, or inoil or gas wells.

The optical fiber 17 may be wound in parallel with a radial support rod20. Radial support rod 20 may be made of a plastic material such asnylon, or other suitable material, and protects optical sensor fiber 17during subsequent handling, including deployment, reeling and extrusion.Additionally, the interstitial volume between the optical fiber 17 andthe radial support rod 20 may be filled with a low modulus material 21such as a thermoplastic elastomer of the type typically used duringstandard cable manufacture to block diffusion of water into the cablestructure, or, alternatively, with a material such as depolymerizedrubber. Low modulus material layer 21 provides support for anysubsequent tape and/or extruded protective layers, as well providing asisolation from external shear stresses on the hydrophone 10 that occurduring or operation of the hydrophone 10.

In one embodiment of the present invention, a layer 22 formed from amaterial, such as Teflon, polyimide or the like, having a lowercoefficient of friction than the jacket of the optical fiber 17 may beapplied over low modulus material layer 21 to ensure radial consistencyof the low modulus material 21. Layer 22 may be wound on the assembly asa layer of tape. Layer 22 is surrounded by a layer 23 formed from a lowshear strength elastomer, such as, for example, polyurethane andsilicone rubber and the like having a hardness on the order ofapproximately 30-40 on the Shore A scale. Acting as a noise reductionmechanism, layer 23 isolates the optical fiber 17 from longitudinallyapplied shear stresses that contribute to acoustic noise within thehydrophone.

The final layer applied to the hydrophone assembly is typically a toughelastomeric outer jacket 24. The outer jacket 24 protects the hydrophone10 from mechanical handling, abrasion, deployment and operationalstresses. Outer jacket 24 may be formed from a variety of suitablematerials including, for example, polyurethane, polyethylene, nitrilerubber, or other materials having the desired physical characteristics.

While an embodiment of the present invention has been described as beingsurrounded by outer jacket 24, in other embodiments, the outer jacket24, and layers 21, 22 and 23 may be omitted. However, such embodimentswill likely need to be disposed within a fluid filled or solid filledtube to protect the hydrophone from damage, as is typical in presentlyavailable towed hydrophone arrays.

FIG. 2 illustrates the details of a hydrophone array 30 including aplurality of hydrophones 55 formed in accordance with the embodiment ofthe present invention described above. In this embodiment of the presentinvention, the plurality of hydrophones may be formed on a continuouscore sub-assembly 31. Prior to winding, one or more fiber Bragg gratings32 are written into an optical fiber 17 (FIG. 1) at predeterminedintervals. This predetermined interval between gratings 32 (FIG. 2)becomes the hydrophone length 33, and may vary depending on the type andwavelength of the signals to be sensed, as well as the sensitivity andimaging capabilities desired. The length of optical fiber between eachfiber Bragg grating is an individual sensing element 55. The opticalfiber 17 is wound around core sub-assembly 31 at a pitch selected tomaintain the required hydrophone acoustic sensitivity and spacing 33based upon acoustic requirements.

Adjacent hydrophones in accordance with the present invention may bebound by gratings having different center wavelengths. For example, inone embodiment, a first hydrophone section is bounded by a gratinghaving a first center wavelength and a second hydrophone section isbounded by a grating having a second, different center wavelength. Anarray of this type provides for wavelength division multiplexing, as thesignals from both arrays will capable of separation and analysis usingsignal processing techniques well known in the art.

One potential problem is the occurrence of mechanical motion betweenadjacent sensors that may reduce the sensitivity of the array.Mechanical coupling of this kind may be reduced, or eliminated, byadding a thin layer of low shear material, such as, for example,polyurethane or the like having a Shore A hardness of approximately30-40, between layer 15 and the acoustic substrate formed by layer 16.In an alternative embodiment, decoupling mechanical motion betweenadjacent sensors may be accomplished by substituting rings of low shearor high loss material 25, such as for example, polyurethane or the likehaving a Shore A hardness of approximately 30-40, in place of theacoustic substrate formed by layer 16 in segments between adjacentsensors, as shown in shown in FIG. 2A.

FIG. 3 illustrates an alternative embodiment of the present inventiondepicting a hydrophone 60 formed in accordance with the descriptionabove, but omitting the outer layers of the hydrophone assemblysurrounding the optical fiber 17. In this embodiment, the hydrophone 60is installed within a liquid-filled hydrophone array of the typecommonly used in towed arrays. Hydrophone 60, which may also includeouter layers and protective covering 24, is installed within a tube 70that is filled with a liquid 65, such as, for example, Isopar or Norpar(ExxonMobil Chemical Company), or other suitable fluid. Tube 70 servesto protect the hydrophone 60, while the fluid 65 acoustically couplesthe hydrophone to the exterior cases to reduce, to the extent possible,attenuation of acoustic signals transferred from the exterior of tube 70to the hydrophone 60, and decouples shear stress between the tube 70 andthe hydrophone 60.

FIG. 4 illustrates yet another embodiment of the present inventiondepicting a hydrophone 80 formed in accordance with the descriptionabove, but omitting the outer layers of the hydrophone assemblysurrounding the optical fiber 17, although there is no requirement toremove the outer layers, and the device would function acceptably if theouter layers were in place. In this embodiment, the hydrophone 80 isinstalled within a hydrophone array having an outer jacket 90.Hydrophone 80 is installed within a tube 90, designed to protect thehydrophone, that is filled with a low shear strength solid fill material85 such as a polymer which may also include voids dispersed throughoutthe polymer to improve acoustic coupling of acoustical signals to thehydrophone 80 to reduce, to the extent possible, attenuation of acousticsignals transferred from the exterior of tube 90 to the hydrophone 80while decoupling shear stress between the tube 90 and the hydrophone 80.

FIG. 5 illustrates one application utilizing a hydrophone array inaccordance with present invention deployed in a bore hole. A boreholehydrophone array 100 in accordance with the present invention may bedeployed in an oil or gas well 105, or any other bore hole such as ageothermal well. A lead cable 110 incorporating a fiber optic fortransmitting signals to and from the array 100 is used to lower thearray 100 using deployment apparatus 115 into the well. Lead cable 110is connected to an acoustic receiver 120, which may contain all of theelectronics and optical components necessary to provide a light beamdown the optic fiber and into the array 100, and also to analyze thephase shifts in the signals returning from the array and to convertthose signals into a form representative of the received acousticsignals that may be displayed, printed or further analyzed.Additionally, interrogator 120 may configured to communicate withadditional processing equipment, such as a computer or computer network.The communications may occur either over wires or other hardconnections, including optical networks, or the communications may occurwirelessly.

It will be apparent to those skilled in the art that the coresub-assembly and outer layers can be manufactured in continuous, onepiece, homogeneous sections, using standard cable manufacturingtechniques, such as extrusion and winding. These sections of the coresub-assembly can be wound with a continuous optical fiber to createhydrophone arrays, with fiber Bragg gratings spaced at appropriateintervals to provide the desired sensor spacing. These continuous, onepiece sections containing the arrays may be wound upon commonlyavailable spools and deployed using deployment equipment commonlyavailable. The novel features of the present invention thus provide asystem and method for providing easily deployable arrays of hydrophonesor geophones that are rugged and capable of withstanding harshenvironments.

While several particular forms of the invention have been illustratedand described, it will be apparent that various modifications can bemade without departing from the spirit and scope of the invention.

1. A continuous, flexible cylindrical device for detecting acousticsignals, comprising: a flexible core including an acoustic substrate; anoptical fiber wound around the acoustic substrate; and at least oneperiodic refractive index perturbation formed in the optical fiber. 2.The device of claim 1, wherein the acoustic substrate contains aplurality of voids.
 3. The device of claim 2, wherein the voids areformed by hollow microspheres.
 4. The device of claim 3, wherein themicrospheres are formed from a compliant material.
 5. The device ofclaim 1, wherein the flexible core includes a hollow tube for providinga passageway through the core.
 6. The device of claim 1, wherein theflexible core includes a strength member for providing tensile strengthto the core to resist stretching or breaking of the core duringdeployment, retrieval or use.
 7. The device of claim 5, wherein theflexible core includes a strength member surrounding the hollow tube forproviding tensile strength to the core to resist stretching or breakingof the core during deployment, retrieval or use.
 8. The device of claim7, further comprising an intermediate jacket disposed between the metaltube and the central strength member.
 9. The device of claim 6, furthercomprising a jacket disposed over the strength member.
 10. The device ofclaim 8, further comprising a jacket disposed over the strength member.11. The device of claim 1, wherein the acoustic substrate includes a anelastomeric material having a selected dynamic property for limiting thesensor frequency response to within a desired range of frequencies. 12.The device of claim 1, wherein the optical fiber is wound under tensionto form at least one optical hydrophone.
 13. The device of claim 12,wherein the optical fiber is wound under tension to form a plurality ofoptical hydrophones, with each of the hydrophones separated by aperiodic refractive index perturbation.
 14. The device of claim 13,wherein the periodic refractive index perturbation is a Bragg grating.15. The device of claim 13, wherein the periodic refractive indexperturbation is a long period grating.
 16. The device of claim 1,further comprising a layer of tape disposed around the acousticsubstrate under the optical fiber.
 17. The device of claim 16, whereinthe tape has a low coefficient of friction relative to a coefficient offriction of the fiber.
 18. The device of claim 16, wherein the tape isformed from Teflon.
 19. The device of claim 1, further comprising afiller rod, the filler rod inter-wound on the acoustic substrate withthe optical fiber such that the filler rod is disposed approximatelyparallel to the optical fiber.
 20. The device of claim 19, wherein thefiller rod is formed from nylon.
 21. The device of claim 19, wherein thefiller rod has a diameter equal to or larger than a diameter of theoptical fiber.
 22. The device of claim 21, wherein the filler rod andoptical fiber are inter-wound around the acoustic substrate so thatthere is a space between adjacent turns of the wound optical fiber, thespace being filled with a compliant material.
 23. The device of claim22, wherein the compliant material is a thermoplastic elastomer.
 24. Thedevice of claim 22, wherein the compliant material is depolymerizedrubber.
 25. The device of claim 23, further comprising a tape layerdisposed around the inter-wound filler rod and optical fiber.
 26. Thedevice of claim 25, wherein the tape layer is formed from a materialhaving a low coefficient of friction relative to a coefficient offriction of the optical fiber.
 27. The device of claim 26, wherein thematerial is Teflon.
 28. The device of claim 26, wherein the material isa polyimide polymer.
 29. The device of claim 1, wherein the opticalfiber includes an external layer formed from an elastomer.
 30. Thedevice of claim 29, wherein the elastomer is void filled.
 31. The deviceof claim 1, further comprising: an outer tube in which the flexible coreand optical fiber are disposed, there also being a space between theouter tube and the flexible core and optical fiber; and a materialdisposed within the space, the material for coupling acoustic signalsfrom the outer tube to the flexible core and optical fiber.
 32. Thedevice of claim 31, wherein the material is a fluid.
 33. The device ofclaim 31, wherein the material is a low shear modulus polymer.
 34. Thedevice of claim 32, wherein the fluid is Isopar.
 35. The device of claim32, wherein the fluid is Norpar.
 36. The device of claim 31, furthercomprising an outer elastomeric layer disposed on the flexible core andoptical fiber, the elastomeric layer including hollow microspheresdispersed through the elastomeric layer to adjust buoyancy.
 37. Thedevice of claim 31, wherein the material is an elastomeric materialhaving a plurality of microspheres dispersed throughout the elastomericmaterial for providing pressure-compensated structural support for theflexible core and optical fiber.
 38. The device of claim 13, furthercomprising a ring of low shear material disposed between at least onepair of adjacent optical hydrophones.
 39. The device of claim 3, whereinthe microspheres are formed from Expancel.